1. Field of the Invention
The present invention relates to actuator servo control systems used in precision positioning of read/write transducers in data storage devices such as hard drives, and in particular to a system and method for linearizing a position error signal to reduce errors.
2. Description of the Related Art
Disk and tape data storage devices are well known in the art. The data is stored as a plurality of data tracks of predetermined format disposed on a recording medium such as a magnetic disk, an optical disk, or magnetic tape. The data is written to and read from the tracks using one or more transducers or read/write heads, which are electrically coupled to signal processing electronics to provide a data transfer path between the media and a requesting system external to the storage device.
The heads are supported in close proximity to the media by a head positioning assembly capable of operating in two distinct modes: track seeking and track following. During track seeking, the heads are moved transversely to the tracks from a current data track to a desired or target track in response to a read or write request from the external system. Track following is the function of maintaining a head in alignment with a track while reading, writing, or merely idling.
In most storage devices, movement of the head positioning assembly is controlled by a closed loop servo system comprising a combination of servo electronics and microcode. Closed loop systems utilize position information obtained from the surface of the storage medium as feedback to perform the seeking and track following functions. Examples of closed loop servo control system are provided in commonly assigned U.S. Pat. Nos. 4,679,103 and 5,404,254. Some disk drive designs hold servo information on a single, dedicated disk surface (i.e., dedicated servo). Other disk drive designs, and most tape drives, provide servo information embedded between the data regions of the storage media (i.e., embedded servo) in the form of a PES field. The PES field is written using an external positioning device known as a pusher or a servowriter, and read with a magnetoresistive read element. Unfortunately, because of manufacturing tolerances, the read and writing elements can vary in width, which can cause the sensed head position to vary non-linearly with the actual head position.
Servo information typically includes a track identifier and a burst pattern which are combined to produce a position signal. The track identifier is commonly in the form of a grey code or track address and is used to uniquely identify the track currently beneath the transducer. The burst pattern produces an analog signal indicative of track type and head offset with respect to the center of the current track. If a quadrature burst pattern is used, the resulting analog signal is demodulated into primary (PESP) and quadrature (PESQ) signals. When the head moves transversely to the tracks during a seek operation, the track identifier and quadrature contributions are combined or xe2x80x9cstitchedxe2x80x9d together to ideally provide a linear position signal.
As storage devices move toward greater track densities, the accuracy of the position signal becomes increasingly important for enabling data storage and retrieval, particularly when using magnetoresistive (MR) heads. MR heads comprise a magnetoresistive read element in combination with an inductive write element. Manufacturing tolerances in the production of MR heads typically causes an offset between the respective centers of the read and write heads which results, on occasion, in the read head servoing at or near the stitch points of the position signal. This is necessary to center the read head to an offset position aligned with the radial position where data has been written. Discontinuities at these junctures may cause relatively large changes in the position signal for very small real head movement, resulting in erratic actuator movement.
Unfortunately, the position signal often includes discontinuities at the stitch points due to PES gain variation. Gain variation is attributable to such factors as write width modulation, servowriter runout, transducer fly height, variations in head width, and gain errors introduced by the automatic gain control (AGC) process itself.
One approach for correcting nonlinearity of the position signal is to include a gain adjustment stage prior to the servo control loop. For example, commonly owned U.S. Pat. No. 4,578,723 addresses nonlinearities in the slope of the position signal in a disk drive due to variations in electromagnetic read head widths. The reference discusses a number of techniques for controlling the AGC output gain. One technique uses the sum (p+q) of contributions from a pair of servo tracks to normalize the position error signal, since this sum corresponds to the full head width and is therefore a constant quantity. (The p+q sum referred to in this reference is equivalent to the sum of A+C in a quadrature burst environment, i.e., the sum of the contributions from bursts positioned to either side of the center of a data track.) The position error signal (which is comparable to PESP in a quadrature environment) is given by (pxe2x88x92q)/(p+q) multiplied by a constant. The problem with this approach is that although it provides some standardization of gain, it does not correct for nonlinearity, errors in the slope of the position error signal or discontinuity at the stitch points of the final position signal.
A second approach discussed in the reference is to add a calibration step at the output of the AGC loop to bring the AGC gain within a predetermined acceptable range. However, this approach also fails to correct for nonlinearities and tends to only be nominally effective at the stitch points.
Another technique proposed by the reference is to include a gain function generating means in the AGC loop to provide, at any radial position of the head, a gain function which is a measurement of the rate of change (i.e. slope) of the position error signals per track of displacement. The gain control loop controls a variable gain amplifier in dependence on the gain function so as to keep the measured rate of change substantially constant. While this approach works on average on a sector-by-sector basis, it leaves open the possibility to discontinuities at the stitch points.
Still another technique is discussed in commonly owned U.S. Pat. No. 5,825,579, issued to Cheung et al, which is hereby incorporated by reference herein. This reference discusses the use of a normalization stage applied to a servo control loop to provide continuity at stitch points. However, while this normalization can reduce discontinuities, it does not improve and will typically increase PES non-linearities near zero track points.
What is needed, therefore, is a system and method for linearizing the PES signal for use in positioning a transducer relative to a storage media surface. What is further needed is for such system and method to be operable with normalization techniques to provide a continuous stitched position signal.
To address the requirements described above, the present invention discloses a method, apparatus, and an article of manufacture for calibrating a position error signal for controlling the position of a read transducer with respect to a storage media having positioning information written at a location thereon.
In one embodiment, the invention is embodied in a method comprising the steps of determining a relationship between a read transducer position and the positioning information by measuring the positioning information while passing the read transducer across the location, determining a measured position error signal from the measured positioning information, and computing a relationship between the measured position error signal and the read transducer position. The invention is also embodied in an apparatus comprising a means for performing the operations described above, and a program storage device tangibly embodying instructions for performing the method steps described above.
As described herein, this calibration can be implemented to minimize linearity errors, while allowing discontinuity removal to be performed by an auxiliary scheme. Alternatively, this calibration can take into account the effect of the normalization procedures used to eliminate discontinuities in the stitched signal, thus reducing the non-linearity of the PES signal after normalization.